The present invention relates generally to a portable telephone, more specifically to a portable telephone set to receive its operation power from either a built-in battery or an external power source.
Conventionally, this type of portable telephone comprises, as illustrated in FIG. 4, an antenna 1, a duplexer 2, a receiver section 10, a transmitter section 4, an external power input terminal 8, a power switch 5, a power selector 6 and a battery section 7. The operation power for the receiver section 10 is supplied either from the battery section 7 or the external power input terminal 8. In case of using the built-in batteries, the operation power for each element in the receiver section 10 is supplied from the battery section 7 by way of the power selector 6, a power switch 5 and a regulated power supply 22. On the other hand, in case of using an external power source, an external power source is connected to the external power input terminal 8. Such external power source is made available to each element in the receiver section 10 again through the power selector 6, the power switch 5 and the regulated power supply 22. It is typical that the voltage V1 of the external power supply is substantially equal to the reference voltage V0 of the built-in batteries included in the battery section 7.
In the conventional example as illustrated in FIG. 4, the operation power for all elements in the receiver section 10 is supplied to a single regulated power supply 22. However, there are instances where a plurality of regulated power supplies are included in the receiver section 10 or a regulated power supply is required to supply operation voltages to both of the receiver section 10 and the transmitter section 4. Also, there are certain instances where no regulated power supply is employed. However, it is common to all of the conventional portable telephone sets that the built-in batteries and the external power source supply operation power of essentially equal voltage to all elements in the receiver section 10.
As described hereinbefore, conventional portable telephone sets are capable of supplying their operation power from either the built-in batteries or the external power supply. However, the portable telephone is mostly operated on the built-in batteries rather than the external power source. It is also true that the portable telephone is frequently used in automobiles or in offices. As the frequency of using the portable telephone in automobiles and offices increases, it is obviously convenient to use the external power source such as car batteries or the AC power supply rather than the built-in batteries in order to avoid time consuming replacement or recharging of the built-in batteries. Especially, in case of using portable telephone in automobiles, it may be used with a booster powered by the external power source to amplify the transmitting output, thereby making it possible to use the portable telephone similar to the car-mounted telephone. An external power supply is generally used as the power source for the portable telephone in such instance.
A synthesized local oscillator is usually used in the portable telephone. Wireless channel switching can be made by simply switching the output frequency from the local oscillator. In the conventional example as illustrated in FIG. 4, the oscillation frequency of a voltage controlled oscillator 14 is switched by controlling at a synthesizer section 15, thereby effecting the wireless or radio channel switching by switching the frequency of first local oscillation power to be supplied to a first frequency mixing section 12. Adjacent channel selectivity is a typical performance of the receiver section of the portable telephone as the wireless communication equipment. Generally, the most influential factor to the channel selectivity is the S/N ratio in the first local oscillation power where S indicates the signal level of the signal to be noted and N indicates the noise level. The S/N ratio in the first local oscillation power is essentially determined by the S/N ratio in the output signal from the voltage controlled oscillator 14 in the conventional example as illustrated in FIG. 4. Other elements in the receiver section 10 will be described later.
In general, the S/N ratio in the voltage controlled oscillator has a strong correlation to the power supply voltage and the current consumption. For example, if designed with 5 volts or lower power supply voltage with 3 mA or lower current consumption, the S/N ratio is apparently inferior to the equipment designed with 10 volts or lower power supply voltage and 10 mA or lower current consumption. Accordingly, the performance of the receiver section using the voltage controlled oscillator of the latter design as the local oscillator is superior in channel selectivity.
Another performance of the portable telephone is a current consumption under the stand-by condition. Assuming that the stored maximum energy of the built-in batteries is constant, the smaller the stand-by current, the longer is the stand-by time of the portable telephone, which is advantageous to the user. Accordingly, reducing the stand-by current is a key factor in designing the portable telephone.
It is, therefore, preferable to reduce current consumption as much as possible to meet the performance requirements of the wireless communication equipment in designing the receiver section of the portable telephone. Channel selectivity is regulated in accordance with the transmission output level. That is, the higher the transmission output level, the more strict are the requirements for the channel selectivity. If the portable telephone is to be used with a booster or the like in a similar manner to the car-mounted telephone (mobile or cellular telephone), a higher performance in channel selectivity is required, similar to the cellular telephone. For this end, it is effective as understood from the above description that a higher voltage and a larger current should be supplied to the voltage controlled oscillator 14 in the receiver section 10.
Unfortunately, in the conventional portable telephone as described hereinbefore, each element in the receiver section is designed to be operated on the power supply voltage V1, essentially similar to the output voltage V0 from the built-in batteries when operated from the external power source. The power supply voltage to be supplied to the voltage controlled oscillator 14 of the portable telephone is essentially the same as that operated on either the built-in batteries or the external power source, thereby making it impossible to optimize the performance of the receiver section 10 by varying the operational conditions of the voltage controlled oscillator 14, depending on the available power sources.